CHAPTER 4. SCREENING ANALYSIS 

TABLE OF CONTENTS 

4.1 INTRODUCTION ................................................... 4-1


4.2 DISCUSSION OF DESIGN OPTIONS ................................... 4-1


4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS ............. 4-3


4.3.1 Conductor Materials ........................................... 4-3


4.3.2 Core Materials ................................................ 4-4


4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS ................. 4-5


4.4.1 Silver as a Conductor Material .................................... 4-6


4.4.2 High-Temperature Superconductors ............................... 4-6


4.4.3 Amorphous Core Material in Stacked Core Configuration ............... 4-7


4.4.4 Carbon Composite Materials for Heat Removal ....................... 4-7


4.4.5 High-Temperature Insulating Material .............................. 4-8


4.4.6 Solid-State (Power Electronics) Technology ......................... 4-9


4.5 SUMMARY OF DESIGN OPTIONS SCREENED OUT ..................... 4-9


LIST OF TABLES 

Table 4.2.1 General Loss-Reduction Interventions for Distribution Transformers ....... 4-3


Table 4.3.1 Design Options Not Screened Out of the Analysis ..................... 4-5


Table 4.5.1 Design Options Screened Out of the Analysis ......................... 4-9


4-i 

 EE-RM/STD-00-550 COMMENT 196
CHAPTER 4. SCREENING ANALYSIS 

4.1 INTRODUCTION 

The purpose of the screening analysis is to identify design options that improve 

distribution transformer efficiency and to determine which options the Department will evaluate 

and which options will be screened out. The Department consults with industry, technical 

experts, and other interested parties in developing a list of design options for consideration. It 

then applies the following set of screening criteria to determine which design options are 

unsuitable for further consideration in the rulemaking (10 CFR Part 430, Subpart C, Appendix A 

at 4(a)(4) and 5(b):

 (1) Technological feasibility. Technologies incorporated in commercial products or in 

working prototypes will be considered technologically feasible;

 (2) Practicability to manufacture, install, and service. If mass production of a technology in 

commercial products and reliable installation and servicing of the technology could be 

achieved on the scale necessary to serve the relevant market at the time of the effective 

date of the standard, then that technology will be considered practicable to manufacture, 

install, and service.

 (3) Adverse impacts on product utility or product availability. If a technology is determined 

to have significant adverse impact on the utility of the product to significant subgroups or 

consumers, or result in the unavailability of any covered product type with performance 

characteristics (including reliability), features, sizes, capacities, and volumes that are 

substantially the same as products generally available in the U.S. at the time, it will not be 

considered further.

 (4) Adverse impacts on health or safety. If it is determined that a technology will have 

significant adverse impacts on health or safety, it will not be considered further. 

This chapter discusses all the design options the Department considered for improving the 

energy efficiency of distribution transformers, and describes how the Department applied the 

screening criteria. 

4.2 DISCUSSION OF DESIGN OPTIONS 

There are several well-established engineering practices and techniques for improving the 

efficiency of a distribution transformer. A transformer design can be made more energy-efficient 

by improving the materials of construction (e.g., better quality core steel or winding material) and 

by modifying the geometric configuration of the core and winding assemblies. 

4-1


 EE-RM/STD-00-550 COMMENT 196
Core and winding losses are not independent variables of transformer design; they are 

linked to each other by the heat they generate and by the physical space they occupy. 

Transformers are designed for a certain temperature rise, resulting from the heat generated by 

transformer losses during operation. The upper boundary on the temperature rise is a design 

constraint, based on industry practice and standards (Institute of Electrical and Electronics 

Engineers IEEE C57.12.00 and C57.12.01). If this temperature limitation is exceeded, it will 

accelerate the aging process of the insulation and reduce the operating life of the transformer. 

In addition to the core and winding assemblies, a transformer has other nonelectromagnetic elements that may constrain the design of a transformer: the electrical insulation, 

insulating media (oil for liquid-immersed transformers and air for dry-type transformers), and the 

enclosure (the tank or case). Once the insulation requirements are set, a transformer design can 

vary both materials and geometry to reduce the losses. 

Making a transformer more efficient (i.e., reducing electrical losses) is a design tradeoff 

between more expensive, lower-loss materials, and the value a customer attaches to those losses. 

For a given efficiency level, the core and winding losses are generally inversely related— reducing 

one usually increases the other. Additionally, at a given loading point and associated efficiency 

level, there can be several viable designs that achieve that efficiency level. The Department found 

that a wide range of designs and efficiencies are technologically feasible using common materials, 

engineering practices, and construction techniques (see Chapter 5). 

Table 4.2.1 presents a general summary of the loss-reduction approaches from which 

transformer design engineers may choose to build more energy-efficient transformers. (This table 

was adapted from Table 2.2 in ORNL report number 6847 published July 1996.)1 For most of 

these approaches, there are clear tradeoffs between no-load (core) losses, load (winding) losses, 

and price. 

Some of the approaches presented in Table 4.2.1 refer to specific technologies (e.g., 

lower-loss core materials, lower-loss conductor materials), while other approaches refer to 

transformer geometry modifications (e.g., core or conductor cross-sectional area). This screening 

analysis considers the materials and technologies that may be used in transformer construction, 

but does not consider geometry or construction modifications such as a larger cross-sectional area 

or different core-stacking techniques. Construction methods and geometric modifications are 

inherent to the design and manufacturing process, and therefore are not a technology or design 

option considered in the screening analysis. These construction methods and geometric 

modifications are controlled by the transformer engineer and/or software design tool to improve 

the efficiency of resultant designs. Thus, they are applied to the designs prepared in the 

engineering analysis (see Chapter 5). 

4-2


 EE-RM/STD-00-550 COMMENT 196
Table 4.2.1 General Loss-Reduction Interventions for Distribution Transformers 

Loss-Reduction Interventions No-Load 

Losses 

Load 

Losses 

Effect on 

Price 

Decrease Core 

Losses 

Use lower-loss core materials Lower No Change* Higher 

Decrease flux density by increasing core 

cross-sectional area 

Lower Higher Higher 

Decrease flux density by decreasing 

volts/turn 

Lower Higher Higher 

Decrease flux path length by decreasing 

conductor cross-sectional area 

Lower Higher Lower 

Decrease Coil 

Losses 

Use lower-loss conductor materials No Change Lower Higher 

Decrease current density by increasing 

conductor cross-sectional area 

Higher Lower Higher 

Decrease current path length by 

decreasing core cross-sectional area 

Higher Lower Lower 

Decrease current path length by 

increasing volts/turn 

Higher Lower Lower 

*

Amorphous-core materials would result in higher load losses because flux density drops, requiring a larger core 

volume. 

4.3 DESIGN OPTIONS NOT SCREENED OUT OF THE ANALYSIS 

The Department considers all distribution transformer design options currently in use by 

distribution transformer manufacturers to be viable. Viable design options include different 

conductor materials for coils and core materials. 

4.3.1 Conductor Materials 

Aluminum and copper are used in current distribution transformer designs and are 

available for use in standard wire sizes and foils. When the two materials are applied in exactly 

the same manner, copper has a higher electrical conductivity and about 40 percent lower resistive 

losses than aluminum. Compared to copper, aluminum is easier to form and work mechanically, 

and can be less expensive. By utilizing aluminum conductor material at a lower current density 

(i.e., larger conductor cross-sectional area), aluminum transformer windings can be built with 

essentially the same load losses as copper. However, aluminum conductors increase core losses 

due to their larger core frames, necessitated by the larger winding space (“core window”) through 

which the windings must pass. It is common for an efficient design option to have copper in the 

high-voltage (HV) windings and aluminum at a lower current density in the low-voltage (LV) 

windings. In these LV windings, aluminum can be used in the form of flat, rolled foils to reduce 

eddy current losses. 

4-3


 EE-RM/STD-00-550 COMMENT 196
Considering the four screening criteria for this technology, the Department did not screen 

out aluminum and copper as conductor materials. These materials are in commercial use today, 

and DOE therefore found them to be technologically feasible. They are obviously practicable to 

manufacture, install, and service because they have been used in mass production for many years 

and are expected to continue to be the primary winding materials for the foreseeable future. 

There are no adverse impacts on consumer utility or reliability associated with the use of these 

conductor materials. Finally, there are no additional adverse impacts on health or safety 

associated with the use of these winding materials. 

4.3.2 Core Materials 

Transformer cores in the past had relatively high losses, since they were fabricated from 

thick laminates of non-oriented, low-silicon, magnetic steels. Modern cores are made with steels 

that incorporate silicon (approximately 2–3 percent) and trace amounts of other elements, are 

cold-rolled to thinner laminations, have improved laminar insulation, and may also be grainoriented or domain-refined (i.e., laser or mechanically scribed steels). 

Amorphous metal (or METGLASa) material allows the construction of a low-loss core. 

Amorphous metal is extremely thin, has high electrical resistivity, and has little or no magnetic 

domain definition. Cores made from this material can exhibit 60–70 percent lower core losses 

than one made of conventional steels. However, amorphous metal material does have some 

drawbacks: It saturates at a lower flux level of 1.57 Tesla versus 2.08 Tesla for conventional 

materials, and it has higher excitation requirements. Amorphous metal material is also fragile and 

requires special handling during the construction process. Additionally, these designs cannot be 

“packed” as effectively into the winding window, causing the designs to have a space factor of 85 

percent versus 95–98 percent for steel core materials, which increases losses. The net effect of 

the lower flux density and higher space factor is a larger core with greater winding (conductor) 

losses and higher production costs. 

The core steels considered in this screening analysis are all those found in commercial use 

today. These include high-silicon electrical steels, both non-oriented hot-rolled and grain-oriented 

cold-rolled, domain-refined grain-oriented electrical steel, and amorphous material (wound core 

designs). The Department considered all of these core materials to be technologically feasible, as 

they are used commercially today (or in the past) by distribution transformer manufacturers at 

varying flux levels and lamination thicknesses. These commercially available high-silicon, coldrolled transformer steels, nominally designated M2-M6, and domain-refined or laser-scribed steels 

are available for use in both stacked- and wound-core configurations. However, at present the 

application of amorphous material is only a viable design option in a wound core. Its 

a

 Registered trademark of Metglas, Inc., a wholly-owned subsidiary of Hitachi Metals, Ltd., Tokyo, Japan. 

4-4 

 EE-RM/STD-00-550 COMMENT 196
manufacturers have not been successful in producing an amorphous product that can be used in a 

stacked-core configuration (discussed in section 4.4.3 of this Chapter). 

These core steels, high-silicon electrical steels, both non-oriented hot-rolled and grainoriented cold-rolled, domain-refined grain-oriented, and amorphous material (wound core 

designs), are considered practicable to manufacture, install, and service, since they are core 

materials that are being used or that have been used by the distribution transformer industry. 

There are no known adverse impacts on consumer utility or reliability, and no known adverse 

impacts on health or safety associated with these core materials. 

Table 4.3.1 summarizes the design options not screened out of the analysis. 

Table 4.3.1 Design Options Not Screened Out of the Analysis 

Design Issue Material 

Conductor Materials for Coils 

Aluminum (wire and sheet) 

Copper (wire and sheet) 

Core Materials 

Cold-Rolled High Silicon (CRHiSi) Steel 

CRHiSi Domain-Refined Steels 

Amorphous Materials in Wound Core 

4.4 DESIGN OPTIONS SCREENED OUT OF THE ANALYSIS 

The Department screened out the following design options from further consideration 

because they do not meet the screening criteria: 

1. silver as a conductor material 

2. high-temperature superconductors 

3. amorphous core material in stacked core configuration 

4. carbon composite materials for heat removal 

5. high-temperature insulating material 

6. solid-state (power electronics) technology 

4-5 

 EE-RM/STD-00-550 COMMENT 196
4.4.1 Silver as a Conductor Material 

The electrical conductivity of silver exceeds that of copper, aluminum, and other normal 

metals at room temperature (25° Celsius). However, silver has a lower melting point, a lower 

tensile strength, and limited availability. The Department found that the use of silver as a 

conductor is technologically feasible, since distribution transformers with silver windings were 

built during World War II because of a war-time shortage of copper. The Department believes 

the use of silver as a conductor would not have any adverse impacts on consumer utility or 

reliability, as it can readily replace copper or aluminum in this application. The Department is also 

not aware of any adverse health or safety impacts associated with the use of this conductor 

material. 

However, the Department screened out silver as a conductor material because it is 

impracticable to manufacture, install, and service. Silver conductor designs are constrained by 

lower operating temperatures (adding to manufacturing complexity) and lower tensile strength 

(material can easily break during manufacturing process). In addition, due to limited availability, 

silver is not feasible to use for mass production on the scale necessary to serve the U.S. 

distribution transformer manufacturing industry. 

Thus, the Department screened silver out from further consideration as a conductor 

material in the analysis due to its impracticability to manufacture, install, and service (criterion 2). 

4.4.2 High-Temperature Superconductors 

A new class of high-temperature superconducting (HTS) materials was discovered in 

1987. These new materials become superconducting at temperatures close to that of liquid 

nitrogen, a readily available coolant that is considerably less expensive than liquid helium, the 

coolant for the previous generation of superconducting materials. After the discovery of these 

materials, research programs were launched worldwide to explore the use of superconducting 

material in power transformers. However, the use of superconductors, both low- and hightemperature, in transformer manufacturing has proven to be an elusive goal. Low-temperature 

superconductors (liquid helium-cooled) are physically possible but not feasible for commercial 

use, since these units are often unable to return to the superconducting state following a high fault 

current condition. For HTS (liquid nitrogen-cooled), a few demonstration power transformers 

have been built, but a prototype distribution transformer has not been constructed. Design 

constraints include unique conductors, unacceptable alternating current variation losses, and 

complex cryogenic support components. Research to overcome these barriers is being conducted, 

some of which is funded by the Department. 

HTS materials were screened out of further consideration in this analysis because they fail 

on two of the four screening criteria. First, the Department does not consider HTS materials to 

be technologically feasible because a HTS distribution transformer has never been built. 

Additionally, due to technical issues associated with HTS power transformers, the Department 

4-6


 EE-RM/STD-00-550 COMMENT 196
does not consider HTS technology a viable loss-reduction technology for distribution 

transformers now or in the foreseeable future. Second, the Department does not consider HTS 

materials to be practicable to manufacture because they are extremely brittle (built of ceramic 

composites), are orders of magnitude more expensive than conventional conductor material, and 

are not mass-produced in a manner that would meet the demands of today’s distribution 

transformer market. Furthermore, they are not reliable in service because they require continuous 

active cooling or they cease to function. With regard to the third screening criterion, the 

Department is not aware of any adverse impacts on customer utility associated with these 

materials. Similarly, the Department is not aware of any adverse impacts on health and safety 

originating from the use of HTS materials. 

Thus, DOE screened HTS materials out of the analysis because of technological 

infeasibility (criterion 1) and impracticability to manufacture, install, and service (criterion 2). 

4.4.3 Amorphous Core Material in Stacked Core Configuration 

As discussed in section 4.3.2, amorphous material is considered a viable core material in a 

wound-core configuration. However, stacked amorphous core material is not presently a viable 

design option for distribution transformers, and has not been demonstrated in a working design by 

a manufacturer. 

The Department screened out stacked core amorphous core material from further 

consideration in the analysis. First, the Department is not aware of any working prototypes that 

use amorphous core material in a stacked core configuration. Thus, the technological feasibility 

of this material has not been demonstrated. Second, the material has not demonstrated its 

practicability with respect to manufacturing, and therefore cannot be assessed as to its ability to 

meet the demand of mass production nor demonstrate its reliability in service. Considering the 

third criterion, the Department is not aware of any adverse impacts on utility or availability to 

consumers associated with this material. Similarly, for the fourth criterion, the Department is not 

aware of any adverse impacts on health and safety from the use of amorphous core material in 

stacked core configuration. 

Thus, DOE screened amorphous core materials in stacked core configuration out of the 

analysis due to technological infeasibility (criterion 1) and impracticability to manufacture, install, 

and service (criterion 2). 

4.4.4 Carbon Composite Materials for Heat Removal 

A new technology that may prove effective in future transformer designs is the use of 

carbon fiber composites for heat removal. These materials offer good heat conduction and 

electrical insulation performance. The U.S. Naval Research Laboratory built small (less than 1 

kVA), high-frequency transformers with this technology and demonstrated a 35 percent size and 

core loss reduction (see U.S. Patent 6,259,347 B1). While these results are impressive, a larger4-7


 EE-RM/STD-00-550 COMMENT 196
scale prototype distribution transformer has not been demonstrated, and if it were technologically 

feasible, it would still be several years away from commercialization. 

The Department assessed carbon composite materials for heat removal from distribution 

transformers, and found the material failed the first screening criterion. These materials for heat 

removal failed the first screening criterion because there are no commercial products or working 

prototypes that incorporate this technology. The Department was not able to assess whether the 

material meets or fails any of the other three screening criteria. Specifically, the Department 

cannot determine whether transformers would be practicable to manufacture, install, and service 

with this new material, since the application of the technology in a distribution transformer design 

has not been determined. Similarly, any potential adverse impacts on consumer utility or 

availability cannot be assessed, and any adverse impacts on health and safety cannot be 

determined at this time. 

Thus, DOE screened carbon composite materials for heat removal out of the analysis due 

to technological infeasibility (criterion 1). 

4.4.5 High-Temperature Insulating Material 

The transformer industry conducts research and development on insulating materials. 

While potentially improving dielectric performance, industry studies this technology to create an 

electrical insulation that can withstand higher operating temperatures, and to create an electrical 

insulation that conducts heat more effectively out of the core-coil assembly. Increasing electrical 

insulation performance would result in smaller effective core and coil volumes, and therefore 

reduce operating losses. 

The Department assessed high-temperature insulating materials, and found that the 

material failed on the first screening criterion. The Department is not aware of any practical hightemperature insulating or composite heat removal material, either in prototype form or in 

commercial products. The Department was not able to assess whether the material meets or fails 

any of the other three screening criteria. Transformers are built today with standard grades of 

insulation (up to 220° Celsius); however, it is uncertain whether higher temperature materials may 

have certain issues that make them impracticable to manufacture, install, or service. Similarly, the 

Department is unable to assess whether there would be any adverse impacts on consumer utility 

or availability due to the lack of a working prototype. Finally, the Department is unable to assess 

whether there would be any adverse impacts on health and safety aspects of a distribution 

transformer because of this material. 

Thus, DOE screened high-temperature insulating materials out of the analysis due to 

technological infeasibility (criterion 1). 

4-8


 EE-RM/STD-00-550 COMMENT 196
4.4.6 Solid-State (Power Electronics) Technology 

The application of solid-state (power electronics) technology to transformers is in the 

early stages of research. A small test transformer was built at Purdue University as a modeling 

exercise, but no distribution transformer prototype has ever been manufactured using this 

technology. 

The Department assessed the feasibility of solid-state (power electronics) technology, and 

found that this technology failed on the first screening criterion. The Department is not aware of 

any solid-state distribution transformer, either in prototype form or in a commercial product. The 

Department was not able to assess whether solid-state transformer technology meets or fails any 

of the remaining screening criteria. Due to the lack of a working prototype, the Department is 

uncertain whether this technology may have certain issues that make them impracticable to 

manufacture, install or service. Similarly, the Department is unable to assess whether there would 

be any adverse impacts on consumer utility or availability associated with this technology. Finally, 

the Department is unable to assess whether there would be any adverse impacts on health and 

safety aspects of a distribution transformer. 

Thus, DOE screened solid-state power electronics transformer technology out of the 

analysis due to technological infeasibility (criterion 1). 

4.5 SUMMARY OF DESIGN OPTIONS SCREENED OUT

 Those design options that DOE screened out from further consideration are listed below 

in Table 4.5.1. The design options that DOE did not screen out of the analysis are listed in Table 

4.3.1. 

Table 4.5.1 Design Options Screened Out of the Analysis 

Design Option Excluded Screening Criteria 

Silver as a Conductor Material Practicability to manufacture, install, and service 

High-Temperature Superconductors Technological feasibility; 

Practicability to manufacture, install, and service 

Amorphous Core Material in Stacked Core 

Configuration 

Technological feasibility; 

Practicability to manufacture, install, and service 

Carbon Composite Materials for Heat Removal Technological feasibility 

High-Temperature Insulating Material Technological feasibility 

Solid-State (Power Electronics) Technology Technological feasibility 

4-9 

 EE-RM/STD-00-550 COMMENT 196
REFERENCES


1.	 ORNL, 1996. Oak Ridge National Laboratory, Determination Analysis of Energy 

Conservation Standards for Distribution Transformers, P.R. Barnes, J.W. Van Dyke, B.W. 

McConnell, S. Das, ORNL report number 6847, Oak Ridge National Laboratory, Oak 

Ridge, Tennessee. Published July 1996. Available online at: 

www.eere.energy.gov/buildings/appliance_standards/ commercial/dist_transformers.html 

4-10


 EE-RM/STD-00-550 COMMENT 196